Convenience foods became increasingly important in the years after World War II as busy lives meant less time for food preparation. Fresh fruits and vegetables were available only seasonally. Frozen foods were entering the market in the 1940s and 1950s. Dehydrated products were available, but they suffered from lengthy preparation times (hardly making them "convenient"), flavor deficiencies, and difficulty in re-hydration.1

In the summer of 1953 research began on the potato flake process at the Eastern Regional Research Center.2 This facility was particularly interested in finding in new methods for processing potatoes that would allow for the use of potatoes with a relatively low solid content, varieties of which were common in the eastern United States. Previously, Idaho Russet Burbanks were used commercially in making dehydrated mashed potatoes because of their high solid content, necessary to obtain good texture when reconstituted.3

Processing potatoes for storage has a long history; the Incas used climate to produce the first dehydrated potatoes. The process entailed an overnight freezing and thawing cycle plus low humidity. The ice crystals that formed forced openings in the cell tissue allowing for liquid to escape. The Incas employed foot power to expel the liquid. The cycle was repeated several times to lower the moisture content and then the marble-sized potatoes were dried for storage. The Incas called it Chuno, and it was a staple used by soldiers; sufficient quantities were dehydrated as well to guard against shortages.4

Europeans first used potatoes for provisioning ships in the 16th century. Then they were introduced into Europe and by the 18th century the potato had become the major food source in a large part of the continent. But the potato had two major drawbacks: it was bulky and it had a relatively short shelf life, at least in comparison to grains. Experiments began in the late 18th century to solve this problem, in part to make the potato more useful as a foodstuff aboard ships. Potato flour was prepared in 1786 and experiments were conducted to find processes for drying potatoes.5 In the end, potato flour and potato starch – first produced in the United States in 1831 – were the main examples of potato processing.6

Potato chips – first called Saratoga chips – appeared in the middle of the 19th century. At first, chips were prepared in the home, accompanied by some small-scale commercial production. Major commercial applications had to await improved peeling and frying techniques. Potato chip production did not become a big business until after World War II.7

The drop in potato consumption in the middle of the last century coupled with increased consumer demand for convenience foods prompted a renewed search for ways of extending the shelf life of potatoes, with emphasis on developing instant mashed potatoes. Ultimately, two processes emerged for producing dehydrated mashed potatoes: granules and flakes. The USDA's Western Regional Research Center focused on developing a potato granule process. Granules were introduced as a home product in the late 1940s and the military showed interest during the Korean War. Granules were best suited to institutional markets; by the 1970s production of granules declined due to decreasing purchases by the military. While granules have a long shelf life, they do not re-hydrate quickly, limiting their usefulness as a convenience food.8

Potato flakes replaced granules as the leading form of dried mashed potatoes.9 The flake process as developed at the ERRC had the inherent advantage of being easily controlled and it used familiar drying equipment. Appearance and flavor quality were high because the entire dehydration process takes less than half a minute. A short drying process imparts a baked potato flavor to the instant mashed potatoes.10

Initial research at the ERRC was conducted under Laboratory Chief Roderick K. Eskew and two chemical engineers, James Cording and Miles J. Willard. In their initial research, Cording and Willard devised a patent to make flakes on a tiny double-drum drier that dried the potatoes in about twenty seconds from 80% moisture to 5%. Speed of drying was the key; earlier drum-dried potatoes were used for paste as they were not edible. Quick drying eliminated the problem of cell rupture – causing a pasty product – which had plagued earlier attempts at instant mashed potatoes.11 Eventually, Cording and Willard, joined in 1954 by John Sullivan, concluded that a single-drum drier was preferable as it yielded a denser flake that was less costly to package.12

Fixing on a quick drying process using a single-drum drier was a major research breakthrough. Ensuing research focused on the appearance and flavor of the reconstituted flakes. In studies conducted in 1954-55, the researchers found that the structure of the potato cell could best be maintained by utilizing what came to be known as the "Philadelphia cook," a three-step controlled cooking sequence.13

After the potatoes are peeled (treating with lye was determined to be the best method), trimmed, and sliced to about 5/8 inch thickness, the first cooking step occurs. This is a pre-cook at a relatively low temperature to prevent the cells from softening but one that was determined to be high enough to gelatinize the starch in the potatoes.14 The optimal temperature was determined to be between 150° F and 165° F for about twenty minutes. Cording in particular noted that at that temperature and time the cell structure remained intact.15 The initial pre-cook produces the potato slices that are no longer crisp but are tougher, resilient, and translucent. Potatoes pre-cooked the requisite temperature and amount of time remain firm, requiring additional steam cooking to soften them before mashing and dehydration. Because the starch has been gelatinized, the potatoes are not sticky when reconstituted; in fact, stopping the pre-cook at the point of gelatinization is critical for preserving the cell structure.16

After the pre-cook, the potatoes are then cooled to stop the cooking process and further harden the cells.17 Cooks have long known that cooling cooked, starchy vegetables in cold water reduces their stickiness. The USDA studies determined that the temperature of the cooling water and the length to time the pre-cooked potatoes needed to be cooled depended on the variety of cultivar used. In general, lower-solids potatoes needed longer cooling times.18

There are several types of steam cookers employed in the final cook, but whatever the method care must be taken to achieve the requisite separation of the potato cells with a minimum of rupture. Overcooking – resulting in such a rupture – leads to poor texture and flavor in the reconstituted mashed potatoes. Cooking time varies from fifteen minutes to one hour, depending on the solids content of the potato.19

Pastiness caused by cell rupture can also be mitigated if the potatoes are mashed immediately following the final cooking. Experiments at the ERRC determined that additives should be incorporated into the mash before drying to improve texture and extend shelf life. One common additive is a dilute solution of sodium bisulfite to retard non-enzymatic browning; another is a monoglyceride emulsifier which also contains antioxidants and which helps to avoid a pasty of rubbery texture.20

The final step in the production of the mashed potatoes is drying. Tests showed that a single-drum drier outfitted with four to six applicator rolls allowed for the production of potato flakes from lower solids type potatoes commonly grown on the East Coast. This is because the single-drum drier produces thicker and more uniform sheets than the double-drum version. In any event, moisture is reduced in the mash to about 6%.21

The processed potatoes come off the drum drier as a continuous sheet. The sheets must then be broken into flakes, a step in which free starch is released at the periphery of the flake. If the flakes are large the broken cells are insignificant; but as the flakes become smaller, the number of broken peripheral cells increases resulting in a difference in texture. Similarly, a high-density flake suffers less damage on breaking than one of low density.22

13 The researchers at the ERRC turned out numerous reports on their progress, several of which discuss the cooking process. See, in particular, Cording, et al., "Advances in the Dehydration of Mashed Potatoes by the Flake Process,"; Willard, et al., "Potato Flakes. A New Form of Dehydrated Mashed Potatoes,"; Cording, et. al., Potato Flakes. A New Form of Dehydrated Mashed Potatoes. Much of the discussion that follows is based on these and other reports and interviews with researchers.

18 See J. Cording, Jr., J.F. Sullivan, and R.K. Eskew, A New Form of Dehydrated Mashed Potatoes. IV. Effects of Cooling After Precooking, U.S. Department of Agriculture, Agricultural Research Service. ARS 73-27.